One hundred percent full employment is unattainable in a market economy over the long run. Such employment is actually undesirable, because a 0% long-run unemployment rate requires a completely inflexible labor market, where laborers are unable to simply quit their current job or leave to find a better one.
According to the general equilibrium model of economics, natural unemployment is equal to the level of unemployment of a labor market at perfect equilibrium. This is the difference between workers who want a job at the current wage rate and those who are willing and able to perform such work.
Under this definition of natural unemployment, it is possible for institutional factors, such as the minimum wage or high degrees of unionization, to increase the natural rate over the long run.
Natural Unemployment